1. Field of the Invention (Technical Field)
The present invention relates to construction methods and materials, generally to methods and materials for constructing buildings, particularly rammed-earth construction, and specifically to a method for erecting post-tensioned rammed earth structures.
2. Background Art
Mankind for millennia has been erecting buildings made from earth, such as mud, sod, and adobe brick. A somewhat more sophisticated, but long-known, method for using earth as a building material is rammed-earth construction, involving the packing of a soil-cement mixture into forms (often wood framed formworks). Ordinary rammed earth structures, however, are vulnerable to certain types loading, particularly the stresses induced by earthquakes and high winds. The present invention is an advance in the art of rammed-earth construction, devised to overcome its observed vulnerabilities.
Conventional rammed-earth construction methods typically involve the erection of parallel vertical forms that are maintained in spaced-apart relation and exteriorly supported. The forms support the wall during its construction. For example, planar forms are known which may be oriented vertically with their generally smooth interior faces in confronting relation, but separated by a predetermined distance. The spaced relation of the forms is maintained during construction by a variety of known types of spacers or “ties,” which extend between the vertical forms and prevent them from moving toward each other any substantial distance less than the predetermined lateral thickness of the wall. Also, the forms normally feature horizontal and vertical reinforcing ribs on their exterior faces to provide structural integrity. The forms are exteriorly supported to prevent them from moving away from each other any substantial distance greater than the predetermined thickness of the wall. The art of form construction in this regard is well known, and form erection methods for rammed-earth construction may borrow from processes and devices long used in the art of concrete construction.
Rammed earth construction, known generally for centuries and increasing once again in popularity as the cost of other types of construction materials and methods rise, is not without problems. The present invention is directed to increasing the ability for rammed-earth building elements, particularly walls, to withstand shear forces that otherwise result in structural failure.
One of the most important questions pertaining to rammed earth construction is its response to earthquakes and high winds. There are currently a variety of different design approaches employed, depending on the seismic zone in which the structure is to be located. For instance, one method employs individual panels of earth that are encased within a skeleton of cast-in-place concrete. Another method uses a continuous solid earth wall crowned with a beam of reinforced concrete. It also has been attempted to reinforce walls with an ordinary unstressed grid of steel reinforcing bars.
Although these solutions may improve the integrity of the walls in seismic zones, they still leave much to be desired. These solutions are not the most efficient and economical use of rammed-earth construction. For example, installation of a traditional grid of reinforcing steel can dramatically slow the erection of a wall. Further, corrosion of steel rebar in an earth wall is a very real potential problem. The pH level of concrete is much higher than that of soil; the lower pH of many soils can lead to corrosion of steel rebar, especially if the soil has a high moisture content (which is the case in humid climates). If steel reinforcing corrodes and becomes inadequate, an earthen wall may fail without any warning under intermittent loading. Potential corrosion of steel rebar thus becomes an important factor to consider when dealing with earthen construction.
Many modern structures are erected in the shape of a full or partial box to improve their resistance to lateral loading—one of the more destructive kinds of loading inflicted by an earthquake. Box structures may be analyzed by components, based on the components' respective contributions to the lateral load resistance of the building. The movement of the ground during an earthquake delivers forces to the building, which are initially applied to the footings or foundation, and then promptly transmitted to the walls and roof. For simplicity of discussion here, it may be generally assumed that the structural loads act either perpendicular or parallel to the walls. The earthquake load is transferred from the floor or roof diaphragms (the diaphragms are merely the floor or roof structures) to the shear walls. “Shear walls” are those walls oriented roughly parallel to the vector of the earthquake force (i.e., the direction of building movement). More technically, any wall that is not perpendicular to the earthquake force vector will receive some component of applied force; the more parallel the wall is to the imposed force, the greater the shear force it must withstand.
In order for the structural box system to stand up to an earthquake or high wind, the floor and/or roof must be well-connected to the walls. If not, the structure may become unstable and collapse during motion. In addition, each wall, floor or roof element must have enough strength to transfer the load it receives; each element is like a link in a chain—if any link breaks, the entire chain fails. “Flexural walls” are those walls generally perpendicular to the direction of motion in an earthquake or wind. Ideally, these walls “lean” on the diaphragm elements during ground motion, thus preventing the walls from falling inward or outward. Flexural walls, if unsound, also may fail, particularly if unable to withstand the tensions that a created in the “bowing” wall.
Rammed earth structures act like such a box structure during an earthquake. Flexural walls bend “out-of-plane” and shear walls bend “in-plane.” To maintain structural stability, the walls must be of adequate strength to carry the inertial forces developed as a result of their own mass, in addition to the externally applied loads. Further, the walls must be adequately interconnected. Rammed-earth walls erected according to simple convention are mostly unable to withstand tensile stresses, compromising their ability to accept loading during strong wind or earthquake. An unreinforced rammed earth wall undergoing flexure or shear stress tends to fail due to its inability to transmit tensile stresses. The present methodology is directed to solving this latter problem, among others.
Additional background information on the art of rammed earth construction generally can be obtained from Paul Graham McHenry, Jr., Adobe and Rammed Earth Buildings—Design and Construction, (University of Arizona Press, 4th ptg. 1989), which is incorporated herein by reference. Also useful is information found at the following websites on the World Wide Web: “Important Facts About Stabilized Earth,” http://www.rammedearthworks.com; “Earth Materials Guidelines,” http://www.greenbuilder.com; and “Rammed Earth Constructions: Transcultural Research in the Sonoran Desert,” http://ag.arizona.edu. Reference also may be had to U.S. Pat. No. 5,021,202 to Novotny, entitled “Method and Apparatus for Constructing Rammed Earth Walls with Integral Cement Jackets.”